Edgewise coils are coils in which the short side of the flat wire is wound vertically onto an inner diameter face. The cross-sectional area of the area is therefore larger than that of coils in which round wire is wound, since the cross-section is angular. Moreover, since they are wound in a single layer, unlike round wire, which is wound in multiple layers, there is little temperature differential between the inside and the outside of the wire, giving the coil excellent heat-releasing properties with little temperature rise. Thanks to these advantages, such coils are therefore suited for use in high-efficiency reactors.
Conventionally known examples of edgewise coils include self-melting edgewise coils in which a self-melting layer is formed comprising a heat-curing resin on the surface of flat wire or flat braided wire, the wire is then wound into a coil which is heated, thereby turning the wire into a single unit (Japan Patent Application Publication No. 2009-200387 A, Japan Patent Application Publication No. 2009-261086 A). In this case “self-melting” means that the wire is simply heated to melt the resin formed on the surface of the wire itself and adhere adjacent segments of wire together, without the need to use any other adhesive or molding resin. Resins which form a self-melting layer include phenol resins, epoxy resins, polyimide resins, resins in which part of the epoxy resin is transformed into phenol resin, and so on.
Compared to conventional art, in which flat wire is wound into a coil and made into a single unit by impregnating it in or molding it with resin, self-melting edgewise coil uses flat wire which has a self-melting layer, making it possible to eliminate the complexity of the steps involved in impregnation, and the risk of damaging the covering of the wire during molding.
Edgewise coils in which flat wire with a self-melting layer is wound are subject to little tightening force (residual stress), and there is therefore little pressure in the direction of adhesion between segments of wire during the melting (i.e., pressure acting in the axial direction of the coil), which makes it impossible to adhere the wire segments together fully simply by heating the coil. Therefore, in the above Patent Literature a technique is proposed whereby adjacent wire segments are adhered together by applying pressure to the wound coil, which has been set in a mold or the like, in the axial direction thereof.
However, in conventional art, pressure is applied to the wound coil in a direct-contact mold, which causes the self-melting layer on the surface of the wire to become attached to the mold, and this is difficult to remove from the mold. Furthermore, since edgewise coil is not used alone but as a component in reactors and other electronic parts in combination with cores, bobbins, and so on, using the self-melt on the coil alone to make it into a single unit and then attempting to combine it with a core or a bobbin increases the manufacturing steps of the final electronic part. For example, the number of manufacturing steps increases by the number of steps involved in adhering the coil to parts other than the coil, e.g., using an adhesive or the like.
The difficulty involved in removing the coil from the mold and the increase in the number of manufacturing steps are present in other coils, and not just limited to edgewise coils.